The present invention relates in general to the field of gasification for carbon-containing fuel, and in particular to a new and useful process and device for gasifying carbon-containing fuel, in particular coal, whereby a portion of the produced synthesis gas is burned to heat a fluidized bed reactor in which the fuel is gasified.
In coal gasification, a distinction is made between "autothermal" and "allothermal" processes. In autothermal gasification, the heat needed to sustain the gas-forming reactions is produced by combustion reactions that take place in the reactor itself. Not just coal and steam, but air and/or oxygen as well, are fed into the reactor, so that incomplete or sub-stoichiometric combustion of the coal occurs as synthesis gas is produced. In practice, this means that a considerable portion, typically about 30-40% of the coal is lost in the production of heat and only the remainder is converted into available product gas. Another disadvantage is that the available gas is loaded with combustion products and, unless pure oxygen is used for processing, the gas is also loaded with a large proportion of ballast material in the form of nitrogen, which diminishes its usefulness and means that the downstream purification and desulfurizing units must be built correspondingly larger in size. Finally, the relatively high temperature produced in the reactor by the combustion reaction is also disadvantageous, because it makes it more difficult to maintain the fluidized bed and may favor the formation of nitrogen oxides (NO.sub.x) harmful to the environment. Also known to the art are autothermal coal gasification processes wherein a portion of the gas produced is recycled to the reactor and again takes part in the reaction (e.g. German Pat. No. 32 23 702).
In allothermal gasification, heat is supplied from an outside source. Apart from the coal dust, essentially only steam is introduced into the reactor to act as a reaction medium and to sustain the fluidized bed, while the reaction heat required for gasification is essentially provided by an exchange of heat by the reactor with a hot heat exchange medium fed in from the outside. In this process, the coal can be converted essentially completely into available gas, and the produced gas is largely free of combustion residues, nitrogen ballast, etc. Furthermore, the temperature in the reactor can be kept at a lower or optimal level for the gas-forming reaction. The problem there, however, is that to heat the heat exchange medium to be fed into the reactor, an additional source of heat and hence a corresponding demand for primary energy is required. The allothermal process is most advantageous if a nuclear reactor is available to act as such an additional source of energy. In such case, a secondary helium cooling circuit heated by the primary helium cooling circuit of the nuclear reactor can serve both as a heat exchange medium for heating up the reactor and also for the production of the steam required in the reactor (see van Heek and Kirchhoff In "HdT-Vortragsveroffentlichungen" (Haus der Technik Lecture Publications), 453, 1982, p. 59).